1. Field of the Invention
The present invention relates to a projection system, and more particularly to a projection system wherein a dichroic filter, which is an element constituting a color corner included in the projection system, has a flat plate type structure, other than a cube prism type structure, thereby being capable of achieving an improvement in color separation characteristics while allowing an easy manufacture thereof, so that the projection system achieves an improvement in productivity.
2. Description of the Prior Art
Generally, modern persons have an increased tendency to take private leisure and recreation. For instance, they have an increased tendency to see movies or other images in their private spaces. In pace with such an increased tendency, active research and development have been made to provide image display devices having a screen of an increased size.
By virtue of such research and development coping with the tendency to provide a screen with an increased size, data projectors, projection TVs, and projection monitors have been proposed, which utilize a projection technique incorporated with the concept of projectors. These devices use an optical projection engine. The configuration of such an optical projection engine will now be described in brief with reference to FIGS. 1 and 2.
FIG. 1 is a schematic side view illustrating the optical projection engine. FIG. 2 is a schematic front view of the optical projection engine, illustrating the configuration of a color corner included in the optical projection engine.
FIGS. 1 and 2 illustrate a rear projection monitor which includes the optical projection engine. The basic principle of this rear projection engine is to project a bright and distinct image onto a screen using a micro (xcexc)-LCD having a high resolution of, for example, an SXGA grade (1,280*1,024), and a high power lamp so that the user can view the image passing through the screen in the rear of the screen.
The constituting elements of the optical projection engine illustrated in FIG. 1 will be described hereinafter.
The lamp mainly includes two portions. One is a bulb for emitting a light beam. The other is a reflector for reflecting the light beam emitted from the bulb. The lamp may be of a parabolic type or an elliptic type in accordance with the configuration of the reflector used.
In the case of a parabolic lamp, light emitted from the bulb is incident onto the reflector which, in turn, reflects the incident light in the form of parallel light beams. On the other hand, in the case of an elliptic lamp, the reflector thereof is designed to focus the light emitted from the bulb. Accordingly, the brightness of the system, to which the parabolic or elliptic lamp is applied, is determined by the amount of light emitted from the lamp used as a light source for the system.
Meanwhile, the light emitted from the lamp may have three wavelength components, that is, components of an ultraviolet (UV) wavelength band, components of a visible wavelength band, and components of an infrared (IR) wavelength band. Among these wavelength components, the wavelength components needed in the system are those of the visible wavelength band. In particular, light components of the UV wavelength band should be removed because they may degrade the performance of optical elements included in the system, plastic lenses, other optical elements, a polarization prism, and LCDs.
Although light components of the IR wavelength band do not result in a direct damage to optical elements, they may cause those optical elements to increase in temperature upon absorbing light of the IR wavelength band. In this case, the optical elements may be degraded in performance.
In order to solve this problem, a heat glass is used. The heat glass is a filter exhibiting a transmittance for the UV and IR wavelength bands relatively lower than that for the visible wavelength band. This heat glass serves to reflect light of wavelength bands, not used, toward the lamp.
The optical projection engine also includes a light pipe. The reason why the light pipe is used is that the light emitted from the lamp has a non-uniform intensity in such a fashion that it exhibits a high intensity at regions near the optical axis thereof while exhibiting intensity gradually reduced at regions spaced away from the optical axis.
When light having such a non-uniform intensity is reflected from an LCD, the image formed on the screen may have a non-uniform brightness. Therefore, the light pipe is used to make the non-uniform light have a uniform intensity as much as possible.
In FIG. 1, the first and second relay lenses are adapted to focus uniform light beams emerging from the light pipe at a desired position. A mirror is arranged between the first and second relay lenses. This mirror serves to change the path of light in order to optimize the space of the system.
The polarization prism is denoted by xe2x80x9cColor Cornerxe2x80x9d in FIG. 2. The light emitted from the lamp is composed of P-wave components and S-wave components. One of these P and S-wave components should be removed because only a single polarized light must be incident onto an LCD which is an image source. To this end, the polarizer is used.
FIG. 3 schematically illustrates the configuration of the color corner shown in FIG. 1 or 2. Typically, the projection system mainly includes an illumination unit, a color separating/synthesizing unit, and a projection unit.
Where the projection system uses reflective LCDs as an imager, the color separating/synthesizing unit has important functions in that it changes the optical path defined between the illumination unit and the projection unit, separates the illumination light into R, G, and B color components, allows those RGB color components to be reflected by the reflective LCDs, or reflects RGB color components respectively received from the reflective LCDs after synthesizing the received RGB color components. To this end, the color corner is used in the projection system.
In FIG. 3, the reference numeral 1 denotes an illumination unit for separating S-polarized light beams from a non-polarized white light, and illuminating the separated light beams to be subsequently reflected, the reference numeral 2 denotes a projection unit for reflecting light contained with an image onto a screen, thereby allowing the image to be formed on the screen, and the reference character CS1 denotes a first color selecting polarization plate for polarizing G (green)-color light beams into P waves while transmitting S-polarized light beams illuminated from the illumination unit 1. The reference numeral 3 denotes a polarized beam splitter for transmitting the P-polarized G-color light components of the light incident thereto after being transmitted through the first color selecting polarization plate CP1 while reflecting the remaining components of the incident light, thereby changing the travel direction of the remaining light components. Also, the reference numeral 4 denotes a dichroic filter for separating B (blue) and R (red)-color components from the light reflected by the polarized beam splitter 3, the reference numeral 5 denotes a reflective LCD for providing an image corresponding to the P-polarized G-color light beams transmitted through the polarized beam splitter 3, the reference numeral 6 denotes a reflective LCD for providing an image corresponding to the B-color light beams reflected by the dichroic filter 4, the reference numeral 7 denotes a reflective LCD for providing an image corresponding to the R-color light beams transmitted through the dichroic filter 4.
Finally, the reference character CS2 denotes a second color selecting polarization plate. When separated R, G, and B-color light beams are reflected by the reflective LCDs 5, 6, and 7 while containing images, respectively, they are changed in polarity. That is, the B and R-color light beams are polarized into P waves, and the G-color light beams are polarized into S waves. To this end, the second color selecting polarization plate polarizes only the G-color components of the composite light incident to the projection unit 2 into P waves while transmitting the remaining components of the incident light as they are, so as to allow all components of the incident light to have the same polarity.
The dichroic filter 4 comprises two 45xc2x0 prisms, and a color separation layer interposed between the prisms and adapted to separate B and R-color components from each other.
Now, the operation of the conventional projection system having the above mentioned configuration will be described in detail.
When an S-polarized light from the illumination unit 1 is incident to the polarized beam splitter 3, the G-color components of the light are changed into P-polarized light beams at the front surface of the polarized beam splitter 3 because the first color selecting polarization plate CP1 adapted to rotate the polarization direction of G-color light beams by 90xc2x0 is attached to the front surface of the polarized beam splitter 3. At this time, the polarized beam splitter 3 reflects S-polarized light beams while transmitting P-polarized light beams.
As a result, only B and R-color light beams are incident to the dichroic filter 4, and selectively transmitted through or reflected by the dichroic filter 4. In this case, the dichroic filter 4 reflects the B-color components of the light incident thereto while transmitting the R-color components of the incident light.
However, the dichroic filter 4 arranged, including 45xc2x0 prisms, at the color corner has a cube prism type structure. By virtue of such a cube prism type structure, the dichroic filter generates a bandwidth difference of about 70 to 80 nm between P and S waves in terms of spectral transmittance characteristics. The spectral transmittance characteristics of the dichroic filter may be sensitive to a variation in angle. For this reason, it is difficult to practically manufacture such a dichroic filter and to manage the specification of the dichroic filter.
Therefore, the present invention has been made in view of the above mentioned problems involved in the prior art, and an object of the invention is to provide a projection system including a dichroic filter having a flat plate type structure, other than a cube prism type structure, thereby being capable of achieving an improvement in color separation characteristics while allowing an easy manufacture of the dichroic filter, and thus, achieving an improvement in productivity.
In accordance with the present invention, this object is accomplished by providing a projection system comprising: an illumination unit for separating an S-polarized light from a non-polarized white light, and illuminating the separated light to be subsequently reflected; a projection unit for reflecting light, contained with an image and incident thereto, onto a screen, thereby allowing the image to be formed on the screen; a first color selecting polarization plate for polarizing a selected one of R, G, and B components of the S-polarized light, illuminated from the illumination unit, into P waves while transmitting the remaining components of the S-polarized light; a polarized beam splitter for transmitting the P-polarized component of the light incident thereto after being transmitted through the first color selecting polarization plate while reflecting the remaining two components of the incident light, thereby changing the travel direction of the remaining light components; a flat plate type dichroic filter for defining different transmission and reflection optical paths for the separated two light components, respectively, thereby individually separating the two light components reflected by the polarized beam splitter; Three reflective LCDs for providing respective images corresponding to the light component transmitted through the polarized beam splitter and the light components separated by the flat plate type dichroic filter; and a second color selecting polarization plate for transmitting, to the projection unit, the separated R, G, and B-color light components reflected by the reflective LCDs in a state containing images while being changed in polarity, after controlling the R, G, and B-color light components to have the same polarity.
Preferably, the flat plate type dichroic filter has a minimum astigmatism and a thickness of 0.5 mm or less.